Light-scattering translucent products made of polycarbonate with different light-scattering additives, and shaped parts produced therefrom, are already known from the prior art.
Thus, for example, EP-A 634 445 discloses light-scattering compositions containing vinyl acrylate-based polymer particles with a core/shell morphology in combination with inorganic particles, especially TiO2.
The use of light-scattering polycarbonate films in flat screens is described in US 2004/0066645. Polyacrylates, PMMA, polytetrafluoroethylenes, polyalkyl trialkoxysiloxanes and mixtures of these components are mentioned here as light-scattering pigments.
Multi-layer sheets are described in EP 1404520, which contain perfluoro-alkylsulfonic acid salts as an antistatic agent.
In US 2004/0228141, antistatic light-scattering PC films are described in thicknesses of 0.025 to 0.5 mm, which contain fluorinated phosphonium sulfonates as antistatic agents.
In JP 11-005241, light-scattering sheets based on PMMA are described, which consist of a base layer with inorganic scattering pigments and a transparent outer layer with an antistatic agent.
However, the diffuser plates known from the prior art exhibit unsatisfactory stability of the color during lifetime with, at the same time, high brightness and, in particular, they do not have adequate antistatic properties.
When these sheets are handled, i.e. sawn and handled during assembly of the flat screen, however, the problem arises that these sheets become statically charged relatively easily and, for this reason, attract dust to their surface particularly strongly. This dust on the surface drastically reduces the optical properties of these sheets. The light density of the backlight units (BLUs) used in the flat screens is markedly reduced as a result. The aim of this invention is therefore to provide diffuser plates exhibiting a reduced electrostatic charge through a reduced surface resistivity, but at the same time, the optical properties of the diffuser plates are not decreased by the use of the corresponding additives.
The suitability of the light-scattering sheets for their suitability as backlight units for LCD flat screens depends on the brightness of the overall system and on the antistatic properties.
In principle, a backlight unit (direct light system) has the construction described below. It generally consists of a housing, in which, depending on the size of the backlight unit, different numbers of fluorescent tubes, so-called CCFLs (cold cathode fluorescent lamps) are arranged. The interior of the housing is provided with a light-reflecting surface. The diffuser plate, which has a thickness of 1 to 3 mm, preferably a thickness of 2 mm, rests on this lighting system. On the diffuser plate there is a set of films, which may have the following functions: light scattering (diffuser films), circular polarizers, focusing of the light in the forward direction by so-called BEF (brightness enhancing film) and linear polarizers. The linear polarizing film lies directly below the LCD display positioned above it.
Completely surprisingly, it has now been found that diffuser plates containing an antistatic agent from the class of the perfluoroalkylsulfonic acid salts or the class of OH-functionalised polypropylene oxides have an unexpectedly high brightness in the BLU described above. This effect is displayed even more strongly in combination with the set of films typically used in a backlight unit (BLU).
In addition, these diffuser plates exhibit markedly lower surface resistivity than the comparative samples without an antistatic agent. This may be demonstrated on the one hand by determining the surface resistivity, but also by evaluating these sheets in accordance with the dust test described in the examples below. Thus, these sheets exhibit a favorable property of low dust attraction from the environment in the assembly of the BLU.